1. Field of the Invention
The invention is in the field of electrical power production from high temperature geothermal energy sources.
2. Description of the Prior Art
The conventional practice for generating electrical power from very high temperature geothermal fluid sources, as for example sources of geothermal fluid on the order of 500.degree. F. or hotter, is to allow the hot geothermal fluid to flow up out of a well under the power of its own flashing steam, to separate the available steam at the surface, and then to directly drive steam turbine means with this separated steam. However, there are only a few relatively isolated geographical locations throughout the world where such very hot geothermal fluid sources are available and which have a sufficiently low mineral and chemical content to enable this conventional practice to be utilized.
On the other hand, there are large resources of very high temperature geothermal brines which represent a potential source of thermal energy for the production of electrical power, but which have a mineral and chemical content that prohibits application of the conventional procedure of letting the fluid flash up through a well and then driving a steam turbine with steam separated therefrom. The principal reason why such high temperature geothermal brine could not be handled in the conventional manner is that if it were allowed to flash up through the well under its own power, the associated temperature drop would result in a substantial precipitation within the well, usually of carbonates and/or silica, and these precipitated minerals cause scaling and clogging in the flash zone within the well. Other problems directly associated with such flashing in the well are the loss of otherwise usable thermal energy in the heat of vaporization, and a substantial reduction in the flow rate of hot geothermal liquid due to the large volume occupied by the flashed steam in the well bore.
The only way presently known to the applicants for controlling such flashing of hot geothermal brine in a well is to pressurize the brine by pumping within the well to maintain the pressure on the brine above the saturated vapor pressure of the brine so as to avoid flashing, in accordance with the principles set forth in U.S. Pat. No. 3,757,516 issued to Barkman C. McCabe. If such pumping could be applied to pressurize the fluid in the well, it would greatly reduce or completely eliminate scaling and plugging, avoid loss of the heat of vaporization, and greatly increase the liquid flow volume by eliminating competition for space within the well between the geothermal liquid and flashed steam.
However, the pressurization method defined in said McCabe U.S. Pat. No. 3,757,516 for preventing flashing in the well bore has heretofore not been adaptable for use in very high temperature geothermal wells, for a number of reasons. Thus, present pump technology is inadequate to cope with geothermal fluid temperatures above about 375.degree. to 400.degree. F. Another difficulty is that with conventional procedures it would be very difficult if not impossible to control the activity within a very high temperature geothermal well sufficiently to enable a pump to be "stripped into" the well. According to the pumping method shown and described in the said McCabe U.S. Pat. No. 3,757,516, the pump is located in the well below the flash zone. While such procedure is feasible for temperatures up to about 400.degree. F., it involves serious difficulties where the geothermal fluid is at substantially higher temperatures. Thus, as indicated in FIG. 5 of the McCabe U.S. Pat. No. 3,757,516, at temperatures substantially in excess of 400.degree. F. the pumping load and corresponding power consumption by the pump increase rapidly so as to become impractical. As indicated in FIG. 6 of the McCabe U.S. Pat. No. 3,757,516, for temperatures substantially in excess of 400.degree. F. the pump would have to be suspended so deep in the well as to make the installation overly expensive and impractical in order to prevent cavitation of the pump.
Even assuming that hot brines from such high temperature geothermal fluid sources could have been delivered to the surface by prior art means despite the foregoing problems, there are still further practical problems which would have prevented the commercial production of electrical power by conventional methods and apparatus from geothermal sources having characteristics inherent to many high temperature geothermal deposits. One such problem is the presence in some high temperature geothermal fluids of a high dissolved content of the incondensable gas carbon dioxide, as for example on the order of about 5 to 15% by volume, precluding the use of a condensing turbine for the direct steam turbine generation of power. Another such problem is that the geothermal fluid from some high temperature deposits has a dissolved silica content that may be too high for utilization of liquid-to-liquid heat exchangers for transfer of the thermal energy to a power fluid cycle for generating power as disclosed in the said McCabe U.S. Pat. No. 3,757,516. A further such problem is that some very hot geothermal brines have such high chloride content that the fluid may be too corrosive for use of direct liquid-to-liquid heat exchangers.
The foregoing problems in attempting to utilize high temperature geothermal brines at the surface all relate to the heat transfer and generating part of the system. There are also difficult problems rating to the reinjection of such geothermal brines back into the formation from which the brine was originally extracted for assuring maximum regenerative capacity of the system and to satisfy current environmental requirements. Thus, the high mineral concentration inherent in some of the high temperature geothermal brines result in the precipitation of substantial quantities of minerals out of the fluid as the fluid cools while being conducted to and passed back into the aquifer through a reinjection well. Such precipitated minerals tend to clog the reinjection well and reduce the permeability of adjacent earth formations, and also tend to clog the pipeline to the reinjection well.
The applicants have found that some high temperature geothermal fields include both a source of very hot geothermal fluid and a source of much cooler geothermal fluid. Thus, a high temperature geothermal field may include a relatively deep source of very hot geothermal fluid, as for example above about 500.degree. F., and a relatively shallow source of much cooler geothermal fluid, as for example below about 300.degree. F. In some hot geothermal fields the cooler fluid source may be near the edge of a high temperature geothermal field, either in the same earth formation as the very hot fluid source or in a different formation. In such cases the flow rates from the two sources may differ, the lower temperature source usually providing a much greater rate of fluid flow than the high temperature source. In such a dual source situation, the high temperature source may produce a brine that is too high in mineral content for power production with conventionl methods, or its flow rate may be too low for economical power production; while the low temperature of the low temperature source may make power production therefrom economically undesirable. Accordingly, even though the two geothermal fluid sources may together represent a large potential source of power, prior art methods of producing power from geothermal sources may not be applicable for any commercial power production from the field.